Ring-Opening Homo- and Copolymerization ofR-Methylene-ε-caprolactone

Sebastian Sinnwell and Helmut Ritter*

Institute of Organic Chemistry and Macromolecular Chemistry II, Heinrich-Heine-UniVersity ofDuesseldorf, UniVersitaetsstrasse 1, D-40225 Duesseldorf, Germany

ReceiVed December 20, 2005; ReVised Manuscript ReceiVed February 20, 2006

ABSTRACT: The ring-opening polymerization ofR-methylene-ε-caprolactone (R-MCL) is presented for thefirst time. Poly(R-MCL) was characterized by1H NMR, GPC, DSC, and MALDI-TOF. The ring-openingcopolymerization ofR-MCL with ε-caprolactone (ε-CL) yields to formation of radically cross-linkable poly(ε-CL)s with melting points in the range of 35-54 °C depending on the copolymer composition.

Introduction

Various exo-methylene O-heterocycles are known to undergoboth free radical and ring-opening polymerization at appropriateconditions.1a,b Unfortunately, up to now, only a few examplesof exo-methylene lactones have been described in the literature.

Akkapeddi et al. were the first who reported about the radicalpolymerization ofR-methylene-γ-butyrolactone (R-MBL), whichcan be regarded as a cyclic analogue of methyl methacrylate.2a,b

Systematic investigations about the radical homo- and copo-lymerization ofR-MBL and its derivatives followed.3a-f Fur-thermore, it was found that the high ring stability ofγ-butyro-lactone, including that ofR-MBL, usually prevents the normalring-opening homopolymerization4 under standard conditionsmainly because of a positive change of polymerization enthalpy(∆Hp

0).5

However, reports about the polymerization of higherR-me-thylenelactones are relatively rare. The main reason for that maybe due to the fact that these unsaturated lactones are difficultto make because of their low ring stability under the conditionsof R-substitution. As an exception, only the ring-openingpolymerization of 2-methylene-4-oxa-12-dodecanolide leadingto radical cross-linkable polyesters has been investigated up tonow.6 Surprisingly, regarding our knowledge, no informationis available in the literature about the polymerization ofR-MCL4 although the corresponding unmodifiedε-caprolactone (ε-CL)-itself is a well-known monomer for various polyester synthesesthat has even found industrial application.7 Additionally, poly-(ε-caprolactone) dimethacrylates have been described to besuitable precursors to create shape memory materials afterradically cross-linking.8

Thus, in this paper we wish to report for the first time aboutthe ring-opening homopolymerization ofR-MCL 4 and thecorresponding copolymerization withε-CL 1, focusing also onthe crystalline character of the samples. We finally describe ourfirst experiments about the free radical copolymerization ofmethylene function ofR-MCL 4 with styrene as comonomer.

Experimental Section

Materials and Characterization. Diisopropylamine (Merck),n-butyllithium (Acros), chlorotrimethylsilane (Acros), chlorometh-ylphenyl sulfide (Aldrich), sodium (metha) periodate (Fluka), andtin(II) 2-ethylhexanoate (Aldrich) were used as received.ε-Capro-

lactone (Fluka) was distilled over calcium hydride and stored undera nitrogen atmosphere. The synthesized polymers were characterizedby 1H NMR spectroscopy using a Bruker Avance DRX 500spectrometer at 500.13 MHz for hydrogen nuclei and 125.99 MHzfor carbon nuclei, using CDCl3 as solvent. Theδ-scale wascalibrated to TMS. Thermal properties were determined using aMettler Toledo DSC822 controller apparatus in a temperature rangebetween-10 and 100°C at a heating rate of 10°C/min. Themelting point values are reported as the average of the peak maximaof the second and the third measurements. Gel permeation chro-matography (GPC) was performed on a Waters system equippedwith a set of three 300× 8 mm2 MZ Gel SDplus columns (with104, 103, and 102 Å porosity) in THF relative to poly(styrene)standards. Matrix-assisted laser desorption-ionization time-of-flightspectrometry (MALDI-TOF) was performed on a Bruker UltraflexTOF mass spectrometer using the linear mode and a 337 nmnitrogen laser. The samples were dissolved in chloroform and mixedwith a dithranol solution. For optical polarization light microscopy

* Corresponding author: Fax+49-(0)211-81-15840; e-mail H.Ritter@uni-duesseldorf.de.

Table 1. Characterization of r-MCL Homo- and Copolymers

entry [ε-CL]/[R-MCL]Mn

a/103

g mol-1 PDa Tmb/°C yield/%

5a 0 1.3 1.7 28.6 845b 2.5 9.5 1.9 34.8 935c 5 13.4 1.8 43.6 805d 10 13.6 1.8 49.5 975e 20 11.3 1.8 51.2 935f 40 10.7 1.9 52.7 925g 60 11.6 1.8 53.6 89

a GPC with poly(styrene) calibration.b DSC.

Scheme 1. (a-c) Synthesis ofr-MCL 4; 9 (d) Preparation ofHomopolymer 5a and of Copolymers 5b-g

2804 Macromolecules2006,39, 2804-2807

10.1021/ma0527146 CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 03/23/2006

Figure 1. 1H NMR spectra of (a)R-MCL 4 and (b) poly(R-MCL) 5a.

Figure 2. (a) MALDI-TOF mass spectrum (linear mode) of thereprecipitated poly(R-MCL-co-ε-CL) 5e. (b) Mass fractions of poly-(R-MCL-co-ε-CL) 5ewith various proportions vinylidene groups as afunction of molar mass (m/z).

Figure 3. DSC scans of poly(R-MCL-co-ε-CL)s5b-g (second heating:-10 to 100°C; 10 °C min-1).

Figure 4. Dependence of the reciprocal melting points on the negativenatural logarithmic mole fraction ofε-CL 1.

Macromolecules, Vol. 39, No. 8, 2006 Polymerization ofR-Methylene-ε-caprolactone 2805

we used an Olympus BH-2 microscope equipped with an OlympusDP12 microscope digital camera system. Polymer samples werecrystallized from solution (10 mg in 2 mL of THF) on a microscopicslide.

Synthesis of r-Methylene-E-caprolactone (r-MCL) 4.R-MCL 4 was prepared fromε-CL 1 according to the procedureof Takeda:9 a colorless oil,Rf ) 0.34 (hexane: AcOEt 2:1) (lit.9 Rf

) 0.34). 1H NMR (500 MHz, CDCl3): δ ) 1.76-1.81 (m, 2H,-OCH2CH2-), 1.86-1.90 (m, 2H, -OCH2CH2CH2-), 2.40(t, 2H,3JHH ) 5.8 Hz,-C(dO)C(dCH2)-CH2-), 4.20 (t, 2H,3JHH

) 4.9 Hz, -OCH2CH2-), 5.43 and 5.67 ppm (each s,1H,C(dCH2)).

Synthesis of Poly(r-methylene-E-caprolactone) 5a and Poly-(r-methylene-E-caprolactone-co-E-caprolactone)s 5b-g. 0.50 g(4.0 mmol) ofR-MCL was mixed with 2 mol % of the catalyst tin2-ethylhexanoate in a 10 mL vial sealed with a septum (forcopolymerizationε-CL was added according to Table 1). Themixture was heated to 130°C for 3 h. After cooling to roomtemperature, the reaction mixture was diluted with 5 mL ofchloroform and poured into 250 mL of a mixture of cold hexane/diethyl ether (1/1). The precipitated polymer was isolated byfiltration and dried under vacuum.1H NMR (500 MHz, CDCl3):δ ) 1.54-1.60 (m, 2H, -OCH2CH2-), 1.68-1.74 (m, 2H,-OCH2CH2CH2-), 2.34 (t, 2H, 3JHH ) 7.4 Hz, -C(dO)C(dCH2)-CH2-), 4.16 (t, 2H,3JHH ) 6.6 Hz, -OCH2CH2-), 5.54and 6.15 ppm (each s, 1H,C(dCH2)).

Synthesis of Poly(r-methylene-E-caprolactone-co-styrene) 6.0.50 g (4.8 mmol) of styrene was mixed with 0.30 g (2.4 mmol) ofR-MCL and 0.023 g (0.144 mmol, 2 mol %) of 2,2′-azobis-(isobutyronitrile) (AIBN). After the addition of 0.25 mL of DMFthe mixture was purged with nitrogen for 10 min. The mixturewas sealed with a septum and heated under stirring to 60°C for 24h. After cooling to room temperature, the reaction mixture wasdiluted with 5 mL of THF and poured into 250 mL of coldmethanol. The precipitated polymer was isolated by filtration anddried under vacuum. Yield: 0.74 g (93%).1H NMR (500 MHz,CDCl3): δ ) 0.8-2.8 (backbone and ring), 3.5-4.3 (-OCH2-),6.4-7.4 ppm (phenyl). FT-IR:ν̃ ) 3082 (w), 3059 (w), 3025 (w),3000 (w), 2974 (w), 2853 (w), 1712 (s), 1601 (w), 1583 (w), 1493(s), 1452 (w), 1391 (w), 1357 (w), 1282 (w), 1221 (w), 1162 (s),1068 (s), 1029 (s), 966 (w), 909 (w), 846 (w), 812 (w), 757 (s),699 cm-1 (ss). DSC:Tg ) 83 °C. GPC: Mn ) 5040 g mol-1, PDI) 2.0.

Results and Discussion

R-Methylen-ε-caprolatone (R-MCL) 4 was synthesized inthree steps (Scheme 1). First,ε-caprolactone1 was O-silylated,followed by a phenylthioalkylation and completed by theoxidative sulfur removal as described in the literature.9 The ring-

opening homo- and copolymerizations ofR-MCL 4 were carriedout by use of tin octoate as transesterification catalyst (Scheme1). This metal catalyst is one of the most common catalysts forthe successful polymerization of lactones and lactides. Theobtained results of polymerization experiments are summarizedin Table 1. The conversion ofR-MCL 4 was estimated by1HNMR spectroscopy. For that, the relative ratio between theintensity of the hydrogen atoms of the double bond in theR-MCL 4 (singlet at 5.43 and 5.67 ppm) was compared to thehydrogen atoms of the methylene group of the polyesters5a-g(singlet at 5.64 and 6.15 ppm). These measurements indicate acomplete conversion ofR-MCL 4 after 3 h. However, theconversion ofR-MCL 4 is slower compared toε-CL 1. Withinthe detection limit of1H NMR spectroscopy, the exo-vinylidenegroup remains intact. Thus, the ratio of the relative integrals ofthe hydrogen signals of the double bond and the integratedε-hydrogen signals are nearly exactly 1:1:2, as expected. Themolecular weight distributions were determined by GPC withpoly(styrene) as standards. It was found that, under the appliedconditions, the achieved number average of the molecularweights for the homopolymer5a is rather low with 1300 gmol-1. To carry out ring-opening copolymerizations, we usedmole fractions ofR-MCL 4 in the mixture withε-CL 1 from0.016 up to 1.0 (Table 1). The achievedMn values of theobtained copolymers5b-g are in the range of 103-104 g mol-1.

To study the molecular composition of poly(R-MCL-co-ε-CL)s5b-g depending on the concentrations of the incorporatedcomonomers, we investigated the mass fractions of5e by useof MALDI-TOF. Therefore, the mass spectra were transformedinto the molar mass distribution curves for various abundancesof incorporations ofR-MCL 4. In Figure 2a, the MALDI-TOFmass spectrum (linear mode) of the reprecipitated poly(R-MCL-co-ε-CL) 5e is shown as a typical example. Figure 2b showsthe complete set of distribution curves obtained for the variousabundances of incorporations ofR-MCL 4. The incorporationof R-MCL 4 extends from zero to five with decreasing frequencyof each distribution. In addition to the linear chains, rings ofunfunctionalized poy(ε-CL) can be identified.

According to DSC measurement, poly(R-MCL) 5a has amelting point of 28.6°C. The relatively low melt enthalpy of220 ( 62 J/mol of poly(R-MCL) 5a indicates a rather lowtendency of crystallization. It was interesting to study the depen-dence of the melting points on the polymer composition. As aresult from our DSC measurements, Figure 3 shows the secondheating segment (-10 to 100°C; 10°C min-1) of the obtained

Figure 5. Spherolitic structures of (a) poly(R-MCL-co-ε-CL) 5b and (b) poly(ε-CL).

2806 Sinnwell and Ritter Macromolecules, Vol. 39, No. 8, 2006

poly(R-MCL-co-ε-CL)s 5b-g. It becomes clearly visible thatan increasing amount ofR-MCL 4 in the copolyester leads toa lowering of the melting point of the material. Regarding theliterature, eq 1 describes the influence of noncrystallizablecomponents on the melting point of a crystalline material.Tm

means the melting point of the polymer,Tm0 is the melting

point for the homopolymer with an infinite molecular weight,and∆Hm is the repeating unit molar heat of fusion.10 Accordingto our results, the substitution of activity (a) by the mole fraction(Xε-CL) is an ideal approximation (eq 2).

As shown in Figure 4, the dependence of the reciprocalmelting point on the mole fraction ofε-CL 1 was all linear withrespect to negative natural logarithmic mole fraction ofε-CL1. Values forTm

0 ) 53.6 °C and ∆Hm ) 14.8 kJ/mol werecalculated from the straight line (y-axis intercept and slope,respectively) and are confirmed by values from literature11 (Tm

0

) 55 °C and∆Hm ) 15.4 kJ/mol).The crystallization behavior of poly(R-MCL-co-ε-CL)s (5b-

g) was additionally investigated by optical polarization lightmicroscopy. As shown in Figure 5, poly(R-MCL-co-ε-CL) (5b)shows only a weak formation of spherolites (Figure 5a)compared to the picture of pure poly(ε-CL) (Figure 5b).

Finally, to evaluate the free radical vinyl polymerizationproperties ofR-MCL 4, a mixture of styrene andR-MCL 4 (2:

1) was kept in DMF at 60°C in the presence of AIBN for 24h to give the copolymer with number-average of molecularweights of 5000 g mol-1 nearly quantitatively (Scheme 2).

Conclusion

It can be confirmed from the above-described results thatR-MCL 4 is a new type of ring-opening monomer characterizedby the existence of two different polymerizable functions. Ring-opening copolymerizations ofR-MCL 4 andε-CL 1 yield novelradically cross-linkable unsaturated polyesters. The meltingpoints are strongly dependent on the polymer compositionaccording to general equations and are in the range of36-54 °C.

References and Notes

(1) (a) Klemm, E.; Schulze, T.Acta Polym.1999, 50, 1-9. (b) Sanda,F.; Endo, T.J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 265-276.

(2) (a) Akkapeddi, M. K.Polymer1979, 20, 1215-1216. (b) Akkapeddi,M. K. Macromolecules1979, 12, 546-551.

(3) (a) v. d. Brink, M.; Smulders, W.; v. Herk, A. M.; German, A. L.J.Polym. Sci., Part A: Polym. Chem.1999, 37, 3804-3816. (b) Trumbo,D. L. Polym. Bull. (Berlin)1995, 35, 265-269. (c) Stansbury, J. W.;Antonucci, J. M.Dent. Mater.1992, 8, 270-273. (d) Trumbo, D. L.Polym. Bull. (Berlin)1991, 26, 271-275. (e) Mitsuru, M.; Takahashi;M.; Imai, Y.; Pittman, C. U.J. Polym. Sci., Polym. Chem. Ed.1982,20, 2819-2828. (f) Manzer, L. E.Appl. Catal., A2004, 272, 249-256.

(4) Odian, G. In Principles of Polymerization, 4th ed.; Wiley-Inter-science: Hoboken, NJ, 2004; p 581.

(5) Duda, A.; Biela, T.; Libiszowski, J.; Penczek, S.; Dubois, P.;Mecerreyes, D.; Jerome, R.Polym. Degrad. Stab.1998, 59, 215-222.

(6) Uyama, H.; Kobayashi, S.; Morita, M.; Habaue, S.; Okamoto, Y.Macromolecules2001, 34, 6554-6556.

(7) Brophy, M. R.; Deasy, P. B. InEncyclopedia of PharmaceuticalTechnology; 1st ed.; Dekker. New York, 1999; Vol. 2, p 1.

(8) Lendlein, A.; Schmidt, A. M.; Schroeter, M.; Langer, R.J. Polym.Sci., Part A: Polym. Chem.2005, 43, 1369-1381.

(9) Takeda, K.; Imaoka, I.; Yoshii, E.Tetrahedron1994, 50, 10839-10848.

(10) Wunderlich, B. InMacromolecular Physics, 1st ed.; Academic Press:New York, 1980; Vol. 3, p 104.

(11) Brandrup, J.; Immergut, E. H.; Grulke, E. A.; Abe, A.; Bloch, D. R.In Polymer Handbook, 4th ed.; Wiley-Interscience: New York, 1999;Vol I, p 44.

MA0527146

Scheme 2. Free Radical Copolymerization ofr-MCL (4) withStyrene

1Tm

- 1

Tm0

) - R∆Hm

ln(a) (1)

1Tm

- 1

Tm0

) - R∆Hm

ln(Xε-CL) (2)

Macromolecules, Vol. 39, No. 8, 2006 Polymerization ofR-Methylene-ε-caprolactone 2807